This invention relates to nuclear fuel reprocessing and is particularly concerned with the separation of Bum from plutonium and neptunium.
Most commercial reprocessing plants use the Purex process, in which the spent fuel is dissolved in nitric acid and the dissolved uranium and plutonium are subsequently extracted from the nitric acid solution into an organic phase of tributyl phosphate (TBP) dissolved in an inert hydrocarbon such as odourless kerosene. The organic phase is then subjected to solvent extraction techniques to partition the uranium from the plutonium.
More particularly, the organic phase is subjected to separation of fission products by solvent extraction and in some cases then to removal of technetium, before the so-called U/Pu split. In the U/Pu split, Pu(IV) is reduced to Pu(III) which is inextractable into the organic phase and therefore follows the aqueous stream while the U remains in the organic stream. Usually, the reducing agent used in the U/Pu split is U(IV). Np(VI) in the solvent stream is also reduced by the U(IV) to Np(IV). Np(IV) is extractable into the solvent and so exits the contactor in the solvent stream with the U product. Hydrazine nitrate is normally used to stabilise the U(IV) and Pu(III) against oxidation by, in particular, HNO2. The unit for carrying out the partitioning of the U and Pu in practice comprises a contactor having a multiplicity of stages, for example six stages might be used in a modem centrifugal contactor.
There are disadvantages with such a process:
hydrazine is catalytically decomposed by Tc(VII) ions
under certain conditions hydrazine can form undesirable oxidation products (e.g. ammonium salts)
U(IV) must be produced in a separate process on plant, thus increasing costs
two reagents are needed
Np is not separated from U so additional downstream processes are needed to remove Np from U.
It is also a disadvantage of cut commercial Purex processes that they use a three cycle flowsheet [(1) the so-called HA cycle in which fission products are separated and the U/Pu split is performed; (2) the UP cycle in which the uranium stream is purified; (3) the PP cycle in which the plutonium stream is purified]. It is therefore desired to develop an Advanced Purex process in which there is a single solvent extraction cycle.
Moreover, neptunium valency control can be a significant problem in Purex reprocessing. Neptunium is present in the Purex process as a mixture of three different valence states: Np(IV), (V) and (VI). Np(IV) and (VI) are both extractable into the solvent phase whereas Np(V) is in-extractable into this phase. In order to direct Np to raffinate streams, Np is normally stabilised in the (V) oxidation state. This is a complex matter, since not only is it the middle oxidation state of three but Np(V) also undergoes competing reactions, such as disproportionation to Np(IV) and (VI) and is oxidised to Np(VI) by nitric acid. Neptunium control is therefore difficult and efficient neptunium control is a major aim of an advanced reprocessing programme. In commercial Purex reprocessing plants, Np is typically separated from uranium during the uranium purification (UP) cycle. Np(IV) is converted to Np(V) and Np(VI) by heating in the aqueous phase in a conditioner at a high temperature. The conditioned liquor is fed to an extract and scrub mixer-settler where the Np(V) is rejected to the aqueous raffinate. Any Np(VI) present in the aqueous feed is reduced to Np(V) by hydroxylamine which is fed to the scrub section of the contactor. In a typical process, two or three mixer-settlers are required to decontaminate the uranium product from Np.
Numerous studies have been conducted to find replacements for the U(V) +hydrazine system with an efficient reductant for Pu(IV) and Np(VI). Amongst the reductants studied have been butyraldehydes, hydroquinones, substituted hydroquinones and substituted hydroxylamines, such as N-methylhydroxylamine and N,N-diethylhydroxylamine (Yu-Keung Sze, Gosselin Y. A, Nucl. Technology, 1983, vol 63, No. 3, pp 431-441; Koltonov V. S, Baranov S. M., Radiokhimiya, 1993, vol 35, No. 6, pp 11-21; Koltunov V., Baranov S., International Conference on Evaluation of Emerging Nuclear Fuel Cycle Systems (Global-95), September 1995, Versailles, France, Proceedings, vol. 1, pp 577-584).
A disadvantage of such known Np and Pu reduction processes is that their kinetics are potentially too slow for the short residence times of centrifugal or other intensified contactors which would be used in a modern Purex reprocessing plant. In particular, it is difficult to find reducing agents that rapidly reduce Pu(IV) and this is even more difficult if centrifugal contractors are to be used.
It has now been found that Np(VI) and Pu(IV) may be co-reduced by oximes (compounds of the formula R2Cxe2x95x90NOH, where R2 is H or an organic substituent, especially alkyl).
The present invention provides a spent fuel reprocessing method in which an organic phase containing Np(VI) is contacted with an oxime. Preferred oximes are aldoximes of the formula R1HCxe2x95x90NOH, where R1 is an organic substituent. The organic substituent is preferably an alkyl group. Suitable organic substituents (in particular alkyl groups) include those having up to 5 carbon atoms and more especially 1, 2 or 3 carbon atoms, of which methyl and propyl are preferred. The invention includes the use of aldoximes with a substituted alkyl group; as substituents may be mentioned HONH- and hydroxy. The most preferred aldoxime is acetaldoxime which has the formula H3CCHxe2x95x90NOH; another preferred aldoxime is butyraldoxime which has the formula H3CCH2CH2CHxe2x95x90NOH. The oxime reduces Np(VI) to Np(V), which may then be backwashed into an aqueous phase.
The organic phase preferably contains U(VI) and Pu(IV) as well as Np(VI), in which case the oxime reduces the Pu(IV) to inextractable Pu(III), which may be backwashed into the aqueous phase together with the Np(V). At least in preferred processes, especially those using acetaldoxime, butyraldoxime or a homologue thereof, no reduction of Np(V) to Np(IV) occurs so no downstream purification of the U product stream is necessary. It is a significant advantage of acetaldoxime that the rate of reduction of Np(V) to Np(IV) is low. Np(V) is reduced according to:             -              ⅆ                  [                      N            ⁢                          xe2x80x83                        ⁢                          p              ⁡                              (                V                )                                              ]                            ⅆ      t        =                    k        ⁡                  [                      N            ⁢                          xe2x80x83                        ⁢                          p              ⁡                              (                V                )                                              ]                    ⁡              [                              CH            3                    ⁢          CHNOH                ]              ⁡          [              HNO        3            ]      
where k=9.2xc3x9710xe2x88x92412.2mol2.2minxe2x88x921 at 50xc2x0 C.
If such purification should be necessary or desirable, the treated solvent (organic) phase may then be contacted with a hydrophilic complexant-reductant (preferably formohydroxamic acid) to complex with any Np(IV) and reduce any Np(VI), which is backwashed into a second aqueous phase (WO 97/30456).
In a preferred class of methods, the organic phase is contacted with the oxime and the Pu(III) plus Np(V) are backwashed in a first contactor unit into the aqueous phase. U as U(VI) remains in the organic phase and may then be backwashed into dilute nitric acid in a subsequent contactor in the usual manner. The contactors are suitably multi-stage contractors.
The invention includes a method of reducing a species selected from Np(VI) and Pu(IV) to, respectively, Np(V) and Pu(III), which method comprises contacting the species with an oxime. Of course, Np(VI) and Pu(W may be co-reduced by contacting the two species in combination with an oxime.